This invention relates to a printing image carrier, more particularly for indirect gravure or pad printing, of a type in which the carrier, having a gravure image, is a foil image carrier.
Printing image carriers in the form of plates have long been known in the art. Such printing image carriers normally consist of hardened high-alloy tool steel, which is absolutely flat and therefore has to be comparatively thick in order to ensure such flatness. This requirement entails comparatively high costs, more particularly in the case of a relatively large image carrier, since a surface bearing the image has to be lapped and hardened. In some cases a material of the image carrier is artificially aged before machining in order to preclude later warping. Because of the high costs, such steel image carriers produced in this manner can only be utilized economically for big runs of 200,000 prints and more. Owing to comparatively large thicknesses of such steel image carrier plates of up to 10 mm and above, material costs, as such, make themselves felt. Furthermore, costs for shipping and stockholding are relatively high because of the large weights and sizes of the image carriers. Since relatively aggressive inks are frequently employed, it is necessary for a corrosion-resistant steel to be employed as the material for the image carriers, this being a further factor increasing material costs.
For small and medium runs, that is to say for runs of a few hundred, up to 50,000 prints, it is possible to use photopolymer image carriers in place of steel image carriers. Such an image carrier as a rule comprises a backing plate of metal having a plastic-coated surface bearing a gravure image in it. This surface coated with plastic is swept by a doctor in order to remove superfluous ink from the gravure image. Since however the surface of plastic is relatively soft, it is extremely sensitive to the doctor sweeping over it as well as to foreign matter. In fact, foreign matter in printing ink or in the form of dust or dirt on the doctor will lead to scratches in the gravure image and will thereby render the image carrier unusable.
Printing equipment with a closed ink and doctor pot (see European patent publication 140,165 B 1) exclusively requires one of the initially mentioned steel plate image carriers, since only this type has the necessary flat surface, which guarantees a clean printed image with such printing equipment. Surface roughness may, at the most, only be up to 2 microns, because otherwise there is a danger of ink leaking out of the doctor pot. A consequence of this would then be that the machine would be fouled with ink within a short time and that clean printing work would no longer be possible. Because of the small runs possible therewith, such a photopolymer image carrier has the disadvantage that a run frequently has to be interrupted in order to change the image carrier. Such replacement of the image carrier leads to short runs and constant attendance of qualified personnel in order to perform such replacements.
The periodical "Keramische Zeitschrift", 41st year, no. 6, 1989, pages 395 ff, describes an image carrier foil for the ceramics industry. The material applied here comprises ground glass or the like, a large quantity thereof being necessary for each application or, respectively, for each transfer of coloring material. Since such ceramic coloring material is applied in a very thick layer, it is necessary for the gravure image in the carrier itself to have a minimum depth of 70 to 80 microns in order to accept the coloring material and the image carrier must have a corresponding thickness of 0.5 mm to 1 mm even to allow the provision of the gravure image and to endow the image carrier with a necessary stiffness. Such thick image carriers are however relatively stiff and, in the case of line gravure images covering large areas, lead to a disadvantage of a substantial hazard that the gravure doctor will sag into image recesses due to its lack of flatness, even though the surface is completely smooth. Relatively thin printing inks, as are utilized in machines of a type with a closed inking system, however flow through gaps with a depth of 1 micron. It is therefore absolutely necessary for the foil itself to have a constant thickness.
German patent publication 3,412,496 A describes a rotary pad printing device with a foil bound thereon. However there are no details of the foil or the dimensions thereof.
Such image carriers however involve the disadvantage that despite a very high quality ground surface, there is surface roughness of several microns. If such image carriers are utilized in printing equipment with doctor pots, there is then a danger of ink gradually leaking from the doctor pot owing to such unevenness of the surface.